Prior-art semiconductor laser devices are classified into the gain waveguide channel type and the refractive index waveguide channel type in terms of the structure of the waveguide thereof. The structure and the characteristics of the types of semiconductor laser devices will be described in the following.
Three types of double heterojunction (DH) semiconductor lasers of the gain waveguide channel type are shown in FIGS. 1-3. In those semiconductor lasers, the current channel is limited narrowly in a stripe 10, 11, 12 for the current injection so that the transverse fundamental mode of the laser oscillation is stabilized and the threshold value of the driving current for the laser excitation is lowered.
Those semiconductor lasers shown in FIGS. 1-3 are fundamentally produced as follows. On a substrate 1 of GaAs, a first cladding layer 2, an active layer 3, a second cladding layer 4, and a cap layer 5 are deposited successively. Then, electrodes 6, 7 are deposited on both surfaces of the semiconductor laser device. The stripe (metal oxide stripe) 10 shown in FIG. 1 is formed by removing a stripe-like region of an oxide layer 13 deposited on the cap layer 5. The stripe (planar stripe) 11 shown in FIG. 2 is a stripe-like diffusion region 14 formed so as to attain the interior of the second cladding layer 4 through the cap layer 5; the diffusion region 14 has charge carriers of the type opposite to that of the cap layer 5. The stripe (inner stripe) 12 shown in FIG. 3 is formed by depositing a current blocking layer 15 on the substrate 1 and by removing a stripe-like region thereof by etching which has charge carriers of the type opposite to that of the substrate 1.
In the excitation of the laser, the stripes 10, 11, 12, each forming the current channel, confine the electric current flowing into the active layer 3 thereinside; the multilayer structure of the GaAs-AlGaAs system having a double heterojunction which consists of the first cladding layer 2, the active layer 3 and the second cladding 4 confines the laser light in the active layer 3. The effective refractive index is nearly constant in the multi-layer structure, and a gain waveguide channel is formed.
FIGS. 4(A), 4(B) show far-field patterns of a semiconductor laser of the type shown in FIG. 1; the far-field pattern of FIG. 4(A) is obtained in the case that the width S of the stripe 10 is as wide as about 10 micrometers while the far-field pattern of FIG. 4(B) is obtained in the case that the width S of the stripe 10 is as narrow as about 4 micrometers. The far-field pattern is defined as the amplitude of the laser light observed away from the laser device in the plane parallel to the double heterojunction, as is well known to those skilled in the art; the zero degree of the angle in FIG. 4(A) or 4(B) corresponds to the center of the stripe 10. When the width S is as narrow as about 4 micrometers (FIG. 4(B)), the far-field pattern has double peaks, while when S becomes as wide as 10 micrometers, a large central peak superposes the abovementioned two peaks. FIG. 4(C) shows an example of the distribution of the longitudinal transverse mode of the laser oscillation of the semiconductor laser of the type shown in FIG. 1. It is apparent that the laser light oscillates in the longitudinal multimode.
The semiconductor laser of the gain waveguide channel type has the following characteristics. (1) The laser light is apt to oscillate in the longitudinal multimode. (2) The beam waist in the direction parallel to the junction lies at a position inside the end surface of the resonator by 20-40 micrometers. (3) The fundamental transverse mode of the laser oscillation is liable to become unstable by the injection current. (4) If the width of the stripe for the current injection is narrowed to an order of about 4 micrometer, the laser light oscillates in the so-called leaky mode, and the far-field pattern in the plane parallel to the junction is changed into the double-peak pattern as shown in FIG. 4(B). As for the output characteristics, the longitudinal mode varies from the multimode to the single mode at a relatively low output power with increase in output power, and the output power is saturated at a higher value by the hole burning phenomena of charge carriers.
FIG. 5 shows a CSP laser, as an example of a DH semiconductor laser of the refractive index waveguide channel type. A groove 16 of the width W is formed on a surface of a substrate 1. Then, a first cladding layer 2, an active layer 3, a second cladding layer 4 and a cap layer 5 are deposited successively on the substrate 1. A channel 17 of the width S for the current injection is formed by diffusing impurities into a stripe-like region of the cap layer 5 above the groove 16 down to the second cladding layer 4.
The thickness of the first cladding layer 2 is made so thin outside the groove 16 that the light excited in the active layer 3 leaks through the first cladding layer 2 to reach to the substrate 1. The effective refractive index of the active layer 3 of the multilayer structure of the DH type is lowered at the outside of the groove 16 by the leak effect of the light so that a refractive index waveguide channel of the light is formed.
FIG. 6(A) and FIG. 6(B) show a far-field pattern in the direction parallel to the junction and an example of the distribution of the longitudinal transverse mode, respectively, of the CSP laser shown in FIG. 5. The far-field pattern shown in FIG. 6(A) has a single peak in contrast to those of a semiconductor laser of the gain waveguide channel type shown in FIGS. 4(A), (B). The longitudinal mode of the laser oscillation shown in FIG. 6(B) consists of a substantially single wavelength in contrast to the longitudinal multimode of a semiconductor laser of the gain waveguide channel type shown in FIG. 4(C).
In general, a semiconductor laser of the refractive index waveguide channel type has the following characteristics. (1) The laser light oscillates in a longitudinal single mode. (2) The beam waist in the direction parallel to the junction lies near the end surface of the resonator. (3) The transverse mode is stable. (4) the far-field pattern has a single peak, and the shift of the peak is small.
The comparison between the laser characteristics mentioned above suggests that a semiconductor laser of the refractive index waveguide channel type is superior to that of the gain waveguide channel type. However, the former has a disadvantage in that longitudinal mode partition noise is liable to arise. This noise is caused by the transitions from the single mode to other modes at different wavelengths. Such transitions are caused when the output of the laser light is changed, when the temperature of the device is changed, and when the output of the laser light is fedback by even a slight amount to the device, for example by the reflection at an optical part of the laser system. The longitudinal mode partition noise has relatively low frequencies from a few MHz to 10 MHz, and makes the signal-to-noise ratio of the output light of a laser decrease down to about 70 dB. Therefore, the existence of the longitudinal mode partition noise has been a large obstacle against the enhancement of the output power, and makes it difficult for a semiconductor laser to be applied to the light source of a video disc player.
It is known that, in a semiconductor laser which oscillates in the longitudinal multimode, the increase in noise owing to the longitudinal mode partition noise and the light feedback is not remarkable. However, a semiconductor laser of the gain waveguide channel type which oscillates in the longitudinal multimode cannot be applied in practical use owing to the abovementioned disadvantages such as the instability of the fundamental transverse mode.
The inventors directed their attention to a possibility that a semiconductor laser having characteristics different from those of prior-art semiconductor lasers will be realized if the current distribution is made narrower than that of the light intensity in the direction parallel to the active layer by narrowing the width of the stripe to an order of 2 micrometers. In such a semiconductor laser, the laser light will be able to oscillate in the longitudinal multimode up to a relative high output power, and in the transverse fundamental mode up to a high output power A semiconductor laser having characteristics mentioned above can be applied to various uses because such noises as the longitudinal mode partition noise and the light feedback noise are negligibly small.
However, a semiconductor laser structure where the distribution of the current is narrower than that of the light is difficult to be realized according to structures employed in prior-art semiconductor lasers. It is difficult to narrow the stripes 10, 11, 12 for the confinement of the injection current, to be less than about 2 micrometers because of such problems as the technical limits of photolithography, side etching and diffusion in the lateral directions.
It is an object of the present invention to provide a semiconductor laser device wherein the laser light can oscillate in the longitudinal multimode and in the transverse fundamental mode up to a high output power.
It is another object of the invention to provide a production method therefor.
A semiconductor laser device according to the invention comprises (1) a first semiconductor layer having a mesa-shaped stripe, (2) a current blocking layer applied on said first semiconductor layer except the top of the mesa-shaped stripe of the first semiconductor layer, (3) a first cladding layer applied on the current blocking layer and on the top of the mesa-shaped stripe of the first semiconductor layer, and having charge carriers of the same type with that of the first semiconductor layer, (4) an active layer applied on the first cladding layer, and (5) a second cladding layer applied on the active layer, and having charge carriers of the type opposite to that of the first cladding layer, wherein the first cladding layer, the active layer and the second cladding layer compose a multilayer structure of the double heterojunction type for laser excitation.
A production method of a semiconductor laser device having an inner stripe according to the invention comprises the steps of (1) forming two parallel grooves on one surface of a substrate, (2) depositing a current blocking layer on the substrate, (3) forming a groove in the current blocking layer, at the bottom of which the flat top of the mesa-shaped region defined between said two grooves on the substrate is exposed, (4) depositing a first cladding layer on the current blocking region including the groove thereof, (5) depositing an active layer on the first cladding layer, and (6) depositing a second cladding layer on the active layer.
An advantage of a semiconductor laser device according to the invention is that the laser a occurs in the longitudinal multimode and in the transverse fundamental mode at the same time up to a high output power.
An advantage of a production method of a semiconductor laser device according to the invention is that an inner stripe having the width less than 2 micrometer can be formed easily.